(1) Field of the Invention
The present invention relates to a method of producing an electric cell wherein a positive electrode and a negative electrode are piled via a separator and, particularly, to a method of producing an electric cell wherein an acidic solution is used as an electrolytic solution and electrically conductive rubber as exterior material.
(2) Description of the Prior Art
Conventionally, metallic materials, whose electrical conductivity and barrier property to liquids are excellent, are used usually as exterior materials for electric cells when an electrolytic solution used in the cell is an alkaline solution or an organic solvent. However, when the electrolytic solution is acidic, metallic materials are not used, except for some noble metals, as exterior materials, since the metallic materials are corroded. Therefore, carbonaceous materials or electrically conductive rubber materials, wherein powder of carbon (carbon filler) is dispersed, are used when an acidic solution is utilized as an electrolytic solution. When electrically conductive rubber is used as exterior material, application of adhesives might be used as assembling method (adhesive method) for an exterior, however, vulcanization binding method is superior from a standpoint of ease of assembling.
Therefore, in the process of producing an electric cell wherein electrically conductive rubber is used as exterior material and acidic solution is utilized as an electrolytic solution, there are two methods of providing electrolytic solution in the electric cell, i.e., that of forming the exterior after containing the electrolytic solution in electrodes or a separator and that of injecting the electrolytic solution through a hole which is formed after forming the exterior of the electric cell.
However, in the method of forming the exterior after containing the electrolytic solution in electrodes or a separator, an electrolytic solution leaks to the surface of vulcanized rubber due to the expansion or boiling of the electrolytic solution by heating during the vulcanization binding process of the rubber, resulting in insufficient vulcanization which causes reduction in reliability of the exterior material. Furthermore, there is a problem of limited amount of the electrolytic solution, because the amount of containing electrolytic solution in the electrodes or the separator is only that which can be impregnated previously before assembling.
And when the hole for injecting the electrolytic solution is formed after forming the exterior, there is a problem of breaking the electrodes due to the difference of hardness between the electrodes and the exterior materials and, furthermore, a problem of reduction in accuracy of processing.
When the hole for injecting the electrolytic solution is previously formed by making a hollow, cutting, or drilling a hole in the rubber, deformation and plugging of the hole may occur by the pressure and heating during the vulcanization binding process of the rubber, resulting in the reduction of the product yield. Furthermore, there is experienced a difficulty in the process of injecting the electrolytic solution because of variable shape of the hole.